Books/resources for preschoolers who are being bullied by friends?

Not sure this fully amounts to “bullying,” but it’s the best word I have for it and is trending that direction. My daughter A is in pre-kindergarten, and since the beginning of the school has befriended two girls, I and O, in her classroom. I and O went to daycare together before preschool and are quite close. (My daughter also has friends from her daycare at the preschool, but not in her classroom.) Things seemed to be going fairly well before winter break, but for the past several weeks, after school when the girls are on the playground (all the parents tend to pick up at the same time and we let them run some energy out before heading home), we’ve started seeing some really clique-ish and nasty behavior from the I and O aimed at my daughter.

This can look like her asking to play with them and them saying they agreed never to play with her; them all playing together but making my daughter the “bad” character in the games and then being mean to her as part of the game; saying mean things to her; and in limited instances being a little physical (within normal bounds of four year olds who are still learning to control their impulses). The other parents are aware and it seems they are trying to address it with their kids at least to an extent (one day was rough enough that the parents of one of the girls had her make multiple “I’m sorry” cards for my daughter, which was very cute and probably more than was necessary). So I’m not looking for ways to talk to the other parents, but instead looking for advice on books or resources I can use with my daughter to help her process what is happening and make choices that make her happy.

There are three main things I want to get across: (1) if someone is being unkind to you, that is not okay; (2) its okay to seek out new friends, even if it’s scary, and (3) if someone says no to playing with you, it’s okay to be disappointed but you have to respect the no. (That is the one place where I do think my daughter is adding to the dynamic — continuing to try to get the girls to play with her after they’ve made it clear they don’t want to play with her.). I don’t expect her to ditch her friends, but I want to support her in understanding that this dynamic is not how it should be in a respectful friendship, and that it’s not happening because of something wrong with her.

I’d also be curious if folks have had similar experiences — would you involve the teacher even though this is technically happening out of school? Or any other advice? It’s also disappointing to see the cliqueishness starting so young. I remember it well, but I don’t remember it really starting until like 2nd grade, an it hurts my heart to see her facing it so early.